Due to the combination of global regulations promoting fuel efficiency and market demand, fuel economy has driven engine builders to adopt changes to design (engines with smaller tolerances, smaller displacement, direct fuel injection, turbochargers, boosted intakes, start/stops, etc.). Additionally, hardware technology including fuel-electric hybrid and idling stop with engine design modification has placed important new performance requirements on motor oils for passenger cars. Not only must the motor oil address these added design effects, the engine oils are also viewed as an area where additional performance may be achieved. Notably, fuel economy performance of a lubricant is affected by both the viscosity of the oil and additive interactions.
Of the two factors, viscosity has long been regarded as resulting in greater friction reduction and fuel economy. Moving to a lower viscosity engine oil has been a recognized strategy to improve vehicle fuel economy. Recently it has been discovered that this trend does not hold as oils are developed with viscosities that are far lower than those considered previously and thus cannot be read across. For example, moving from an SAE viscosity grade 10W-30 oil to a 5W-30 viscosity oil results in the expected improvement in fuel economy when utilizing the same chemistry in the formulation, but moving to 0W-20 or lower has not demonstrated this trend. One explanation is that this is the result of increased friction in what are known as boundary lubrication situations. These boundary lubrication situations are found when an engine is running at low speed and high temperature. The lower viscosity oils may be less able to maintain separation between moving parts in the engine resulting in increased friction and lowering fuel economy. In addition, as lubricants become thinner, concerns about engine wear increase.
The use of appropriate additive systems is becoming increasingly important. Additives in lubricants often include polar functional groups which will draw the additive to the metal surfaces in an engine. As a result of this interaction, many additives are known to modify the friction performance of a lubricant. Some additives, like detergents, are known to have a negative effect on fuel economy by increasing friction. Balancing the interactions of the additives in the lubricant; and the benefits/potential drawbacks of lowered viscosity is a challenge for today's formulators.
Herein, it is has been shown that certain combinations of detergents and vicinal diol friction modifiers have been discovered which show increased fuel economy benefits in conventional oil and more particularly low viscosity oils of lubricating viscosity. These benefits have been demonstrated through both bench and engine testing.
Vicinal diols are known in the art to be employed in lubricating oils. U.S. Pat. No. 4,406,803 teaches the use of C10-C30 alkane 1,2-diols as friction modifiers in lubricants for internal combustion engines. U.S. Pat. No. 4,331,222 teaches the use of C8-C28 alkane 1,2-diols in functional fluids, particularly those for tractors, to reduce brake noise. JP 2000-017283 teaches the use of greater than C5 alkane 1,2-diols as lubricity agents. JP 2000-273481 teaches the combination of C14-C22 alkane 1,2-diols with a detergent having total base number greater than 60 in a base oil with viscosity index of 80-150 for lubrication. WO 2010/115864 teaches the use of C10-C24 diols in functional fluids particularly for wet brakes. WO 2011/007643 teaches the combination of alkane or alkene 1,2-diols and zinc dithiophosphates in lubricants for improved fuel economy. The class of friction modifiers that includes alkane/alkene 1,2-diols has been in use for decades. However, none of the lubricants previously described address the problem of friction modification in very low viscosity engine oils.